The invention relates to wireless telecommunication systems employing ATM technique (Asynchronous Transfer Mode) in transmission systems.
In mobile networks, radio interfaces have conventionally been narrowband. Mobile network transmission systems have conventionally been implemented with circuit-switched connections using a star or tree network configuration. Examples of prior art mobile systems are the pan-European digital mobile communication system GSM and the Personal Communications System PCS (standard PN-3343) developed in the USA, the network architecture of which is illustrated in FIG. 1. A set of base station systems BSS (a radio system RS in the PCS system) is connected to a mobile switching centre 10 (a Mobile services switching centre MSC in the GSM system, a Personal services switching centre in the PCS system). The base station system BSS comprises base station controllers BSC and base stations BTS. Mobile stations MS communicate over a radio path with the base stations BTS. There is a GSM-based interface A or a PCS-specific interface A between the mobile switching centre 10 and the base station controller BSC. interface A have originally been built for a PCM transmission system or the like. FIG. 2 illustrates an interface A signalling protocol model between the mobile switching centre 10 and the base station system BSS (or the radio system RS).
Interface A is based on an ANSI/CCITT signalling system number 7 (SS7) comprising several layers: an SCCP, an MTP and a physical layer. When a digital PCM link is used between the BSC and the mobile switching centre 10, the signalling of the physical layer is transferred in one or more time slots of 56 or 64 kbit/s. The following higher layers at interface A are an MTP (message transfer part) and an SCCP (signalling connection and control part). The MTP and the SCCP are used to support signalling messages between the mobile switching centre 10 and the BSS.
At present one SCCP user function called a BSSAP (BSS application part) or a RSAP (radio system application part) is determined in the PCS system. The BSSAP (or RSAP) uses one signalling connection for each active MS comprising signalling transactions for transferring layer 3 messages. The BSSAP (RSAP) user function is further divided into two separate functions:
a DTAP (direct transfer application part) is used to transfer call control and mobility management messages between the mobile switching centre 10 and the MS. The BSS does not interpret the DTAP information carried in these messages. Specification GSM 08.06 comprises more details associated with handling DTAP messages in the BSS, multiplexing messages to relevant signalling channels of the radio path and employing SCCP services. Layer 3 protocols for information exchange between MS-MSC are described in series 4 of the GSM specifications.
a BSSMAP (BSS management application sub-part) or a RSMAP (radio system application sub-part) in the PCS system supports other procedures between the mobile switching centre 10 and the BSS relating to the MS (handover control 14 ms) or a cell within the BSS or the entire BSS. In other words, the BSSMAP (RSMAP) supports all the procedures between the mobile switching centre 10 and the BSS (RS) requiring interpretation and handling of information associated with individual calls and resource management.
Discrimination associated with the two sublayers DTAP and BSSMAP (RSMAP) is performed in an intermediate layer between the SCCP and layer 3. The intermediate layer is called a distribution sublayer or a distribution function. The layer 3 DTAP and BSSMAP (RSMAP) messages transferred between the mobile switching centre 10 and the BSS are included in user data fields of the SCCP messages. The user data field also comprises a distribution data unit that the distribution function uses for distributing messages between the BSSMAP and the DTAP functions and for distributing/multiplexing the DTAP messages into different access points in a radio link layer.
To increase the capacity and flexibility of transmission systems the use of different broadband packet-switched transmission systems has also been proposed e.g. in WO 9319559, WO 9400959 and EP 0366342 for mobile services networks. EP0426269 describes a mobile system in which base stations are connected by routers to ATM network switches. Virtual connections controlled by the base stations are established between the base stations through the ATM network. Elementary mobility management is based on routing tables maintained at base stations and in ATM switches and updated as subscribers move. GB 2268359 and EP 679042 describe an ATM access network comprising permanent ATM virtual connections (for speeding up call switching) between base stations and a mobile network interface, the connections being allocated call by call.
A possible future development trend is mobile systems that have a broadband radio interface. Then the transmission system of the mobile system also has to be broadband, while a potential alternative is ATM technique.
At present third generation mobile systems, such as Universal Mobile Communication System (UMTS) and Future Public Land Mobile Telecommunication System (FPLMTS) later renamed as IMT-2000 (International Mobile Telecommunication 2000), are being developed. The UMTS is being standardized in ETSI (European Telecommunication Standards Institute) whereas ITU (International Telecommunication Union) is standardizing the IMT-2000 system. These future systems are basically very alike. In the following the UMTS system will be described in greater detail.
According to the present view the UMTS consists of two or three parts illustrated in FIG. 3: a UMTS access network (or a UMTS base station system, UMTS-BSS) and a core network 2, 3, 4 and 5. Below the UMTS access network is generally also referred to as a radio access network. The UMTS access network is mainly responsible for matters associated with the radio path i.e. offers the core network a radio access needed for wireless operations. The core network 2, 3, 4 or 5 is a conventional or future telecommunication network modified to efficiently utilize the UMTS access network in wireless communication. Telecommunication networks that are thought to be suitable core networks are second generation mobile systems, such as GSM (Global System for Mobile Communication), ISDN (Integrated Services Digital Network), B-ISDN (Broadband integrated Services Digital Network), PDN (Packet Data Network), ATM (Asynchronous Transfer Mode) etc. One of the most probable transmission techniques in access network is the ATM.
The ATM transmission technique is a switching and multiplexing solution particularly relating to a data link layer (i.e. OSI Layer 2, hereinafter referred to as an ATM layer), enabling the implementation of a connectionoriented packet network in the B-ISDN networks (Broadband Integrated Services Digital Network).
In ATM data transmission the end user""s data traffic is carried from a source to a destination through virtual connections. Data is transferred over switches of the network in standard-size packets called ATM cells. An ATM cell comprises a header, the main object of which is to identify a connection number for a sequence of cells forming a virtual channel for a particular call. A physical layer (i.e. OSI Layer 1) may comprise several virtual paths multiplexed in the ATM layer. The virtual paths are identified by a Virtual Path Identifier (VPI). Each virtual path may comprise a number of virtual channels identified by a Virtual Channel Identifier (VCI). The ATM cell comprises indirectly information on the receiver""s address. each cell thus being an independent data transmission unit. Above the ATM layer there are the procedures of ATM Adaptation Layer (AAL) which adapt the ATM layer to the higher layers.
The ATM is a connection-oriented traffic technique, but since there is no connection before it is established, a connection establishment request has to be routed from a source through the ATM network to a destination approximately in the same way as packets are routed in packet-switched networks. After the connection has been established the packets travel along the same virtual path during the connection.
It would be preferable if the mobile systems using ATM transmission systems could use components and protocols of available mobile systems as much as possible.
However, the introduction of ATM technique as the transmission technique of mobile networks brings about changes, for example, in the protocol structure of interface A shown in FIG. 2, as the physical layer does not comprise a PCM link but an ATM layer and higher layers referred to as AAL layers supporting the ATM layer. FIG. 4 shows a known protocol stack formed by existing protocols. In other words, the protocol layers BSSAP, SCCP and MTP3 described above with reference to FIG. 2 are as such connected to the AAL layer according to ITU-T standards and supporting an MPT layer 3 (MPT3). For SSCF (Service Specific Coordination Function) signalling the highest AAL layer is at a network node interface (NNI) defined in ITU-T standard Q.2140. One SSCF user application is MTP3, to which the SSCF offers an adaptation to a lower AAL layer SSCOP (Service Specific Connection Oriented Protocol). The SSCOP is defined in ITU-T standard Q.2110. One of the objects of the SSCOP is the control (establishment, release, resynchronization) of SSCOP connections. A CPCS (Common Part Convergence Sublayer) and a SAR (Segmentation And Reassembly) are below a SSCOP layer. The ATM layer and the physical layer are at the bottom.
An object of the present invention is to provide a more optimal protocol solution for ATM-based transmission systems in wireless telecommunication systems.
This is achieved with a signalling arrangement between network elements in a wireless telecommunication system having an ATM-based transmission system, the arrangement comprising a protocol stack which comprises on top a layer 3 user function for signalling between a wireless mobile station (MS) and a network, an ATM adaptation layer, an ATM layer and a physical layer. In accordance with the invention the ATM adaptation layer comprises on top a service-specified coordination function (SSCF) immediately between said higher user function and a service-specified connection-oriented protocol (SSCOP), said service-specified coordination function comprising as functions the adaptation of the user function to the service-specified connection-oriented protocol and the establishment, discrimination and release of the signalling connections of the ATM adaptation layers needed for conveying signalling messages of the layer 3 user function.
In the invention, a protocol stack comprises a new SCCF located between the SSCOP and the user function, e.g. BSSAP or RSAP, while the SCCP, MTP3 and the SSCF according to ITU-T Q.2140 present in the prior art protocol stack, are omitted. The new SSCF comprises as functions the adaptation of the user function to the SSCOP and the discrimination and control of various connections previously performed in the SCCP. The remaining AAL sublayers SSCOP, CPCS and SAR are in accordance with ITU-T standards, as well as the ATM layer and the physical layer.
The invention is based on the observation that the prior art protocol stack includes two partly overlapping connection control mechanisms, SCCP and SSCOP. Error detection, receipt confirmation and flow control are not needed in the SCCP, since the SSCOP provides these functions. The SCCP segmenting/reassembling function is not either needed, since the SAR provides the segmenting and reassembling needed. Such a double functionality does not seem sensible as the most important SCCP part to be used would be the ability to discriminate several connections using reference numbers given to the connections. This minimal advantage that the SCCP offers is not enough to entitle the implementation of the SCCP and MTP3. Omitting MTP3 also renders the SSCF according to ITU-T Q.2140 unnecessary. All these layers are efficiently replaced with a new SSCF according to the invention in which said control of several connections using reference numbers and the user function/SSCOP adaptation are implemented.
A simpler protocol stack that is easier to implement is achieved with the invention. The efficiency of the protocol stack increases as the number of layers decreases. This is, for example, due to the fact that removing unnecessary layers and overlapping functions reduce the number of header information connected with the original user message, thus shortening the message length. This in turn leads to a more efficient utilization of a transmission band in the transmission system.